Numerous mechanical devices have been developed for supporting the walls of mine shafts and the ceilings of mine tunnels. Generally, such devices are placed in holes drilled in an ore body and an expansion device actuated to afix the device at the end of the drill hole. This is readily accomplished in a bolt device by tightening a nut against a plate thereby shortening the length of the bolt and expanding a gripping device against the drill hole. Such rock bolts support the ceiling and walls in a mine by absorbing stress that may be generated when cracks develop in the rock layers. Rock bolts are limited in their effectiveness by the length of bolt that can be extended into the drill hole. That is, due to the relative inflexibility of a metal rod, maximum bolt lengths are determined by the height and/or width of the tunnel.
In many instances, it is highly desirable to provide support for mine ceilings by providing an anchorage many meters removed from the tunnel. To illustrate, where an unstable geologic formation is known to exist in the vicinity of the tunnel, an anchorage in solid ground may require the use of cables as long as about thirty meters. Needless to say, rock bolts are of little use in such situations.
In some instances, a flexible cable can be used to secure such unstable ground; however, it is necessary in methods such as those disclosed in U.S. Pat. No. 3,971,226 to extend the cable through drill holes connecting different levels of the mine with conventional cable fastening devices. This solution is not entirely satisfactory, since the distance between mine levels may be well in excess of 100 meters and drilling holes this distance within a mine is not generally economically feasible.
Another method for fastening a cable within a drill hole makes use of an explosive charge to spread the end of a cable at the end of a bore hole (U.S. Pat. No. 3,389,561), followed by filling the hole with grout. Also, along this same line, cables are commonly introduced to the drill hole without any expandable end fitting and the void between cable and drill hole filled with grout. Both of these methods are not entirely satisfactory since they rely on filling of a void space. Since cracks may exist in the geologic structure, the grout may follow a crack in the ore body rather than the void between cable and drill hole. In the event that this occurs, an ineffective anchorage is obtained.
Still another cable anchoring device, described in U.S. Pat. No. 2,970,444, cannot be readily assembled with simple hand tools and in fact is shown to use welding during assembly. Furthermore, the device is explosively actuated within the drill hole and therefore requires special handling and personnel.